tracing: Fix very unlikely race of registering two stat tracers

commit dfb6cd1e654315168e36d947471bd2a0ccd834ae upstream.

Looking through old emails in my INBOX, I came across a patch from Luis
Henriques that attempted to fix a race of two stat tracers registering the
same stat trace (extremely unlikely, as this is done in the kernel, and
probably doesn't even exist). The submitted patch wasn't quite right as it
needed to deal with clean up a bit better (if two stat tracers were the
same, it would have the same files).

But to make the code cleaner, all we needed to do is to keep the
all_stat_sessions_mutex held for most of the registering function.

Link: http://lkml.kernel.org/r/1410299375-20068-1-git-send-email-luis.henriques@canonical.com
Fixes: 002bb86d8d42f ("tracing/ftrace: separate events tracing and stats tracing engine")
Reported-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/trace/trace_stat.c b/kernel/trace/trace_stat.c
index 7af67360b3307413865e2ed0719ea63894f01016..4f2f38e4726d0e739c61ff87a4d72adc324d3a96 100644 (file)
--- a/kernel/trace/trace_stat.c
+++ b/kernel/trace/trace_stat.c
@@ -302,7 +302,7 @@ static int init_stat_file(struct stat_session *session)
 int register_stat_tracer(struct tracer_stat *trace)
 {
        struct stat_session *session, *node;
-       int ret;
+       int ret = -EINVAL;
 
        if (!trace)
                return -EINVAL;
@@ -313,17 +313,15 @@ int register_stat_tracer(struct tracer_stat *trace)
        /* Already registered? */
        mutex_lock(&all_stat_sessions_mutex);
        list_for_each_entry(node, &all_stat_sessions, session_list) {
-               if (node->ts == trace) {
-                       mutex_unlock(&all_stat_sessions_mutex);
-                       return -EINVAL;
-               }
+               if (node->ts == trace)
+                       goto out;
        }
-       mutex_unlock(&all_stat_sessions_mutex);
 
+       ret = -ENOMEM;
        /* Init the session */
        session = kzalloc(sizeof(*session), GFP_KERNEL);
        if (!session)
-               return -ENOMEM;
+               goto out;
 
        session->ts = trace;
        INIT_LIST_HEAD(&session->session_list);
@@ -332,15 +330,16 @@ int register_stat_tracer(struct tracer_stat *trace)
        ret = init_stat_file(session);
        if (ret) {
                destroy_session(session);
-               return ret;
+               goto out;
        }
 
+       ret = 0;
        /* Register */
-       mutex_lock(&all_stat_sessions_mutex);
        list_add_tail(&session->session_list, &all_stat_sessions);
+ out:
        mutex_unlock(&all_stat_sessions_mutex);
 
-       return 0;
+       return ret;
 }
 
 void unregister_stat_tracer(struct tracer_stat *trace)